Using ultrastructural, histological, and electrophysiological techniques, we propose to examine the relationship between aging and the regeneration of ablated CNS neurons in vertebrates and invertebrates. In particular, we wish to determine if the ability to regenerate nerve cell bodies is associated with an ability to add new nerve cells during ontogeny and how this ability is related to the age or sex or the organism. We hope to obtain data to help explain why adult mammals (including man) do not regenerate nerve cell bodies. Such data could lead to ways to stimulate CNS regeneration or to a more certain recognition that such regeneration is theoretically impossible. In either case, such data should provide a more justifiable basis for the treatment of elderly patients with CNS lesions.